Vital signs or a person, for example the heart rate, the respiration rate or the blood oxygen saturation, serve as indicators of the current state of a person and as powerful predictors of serious medical events. For this reason, vital signs are extensively monitored in impatient or outpatient care settings, at home or in further health, leisure and fitness settings.
One way of measuring vital signs is plethysmography. Plethysmography generally refers to the measurement of volume changes of an organ or a body part and in particular to the detection of volume changes due to a cardio-vascular pulse wave travelling through the body of a subject with every heartbeat. Plethysmography signals can be determined from a patient's skin by means of a remote photo-plethysmography measurement. Further, it is possible to derive a respiration signal from the photo-plethysmography signal received from the patient's skin color changes as described in WO 2011/135440 A1.
Further, camera-based remote monitoring methods for detecting vital signs of a patient, e.g. the respiration rate by means of motion pattern detection are known from WO 2012/140531 A1.
Since the camera-based photo-plethysmography is based upon skin detection of the patient to be measured and since the skin is not always visible in a field of view due to movement of the patient, the measurement of the respiration rate on the basis of the photo-plethysmography signal is not always reliable.
Further, since the area to be measured, e.g. the chest of the patient can be located freely in the field of view of the camera, it is difficult to determine the relevant portion from which the vital signs should be derived and the region of interest has to be selected in advance, wherein a movement of the subject may lead to faulty measurements.
The traditional identification of the region of interest in general is based on detection of human beings, e.g. a face or a chest or by using background segmentation. In order to identify a human being for measuring vital signs such as a pulse or respiration rate from a region of interest, US 2009/0141124 A1 suggest to detect a contour in an infrared video segment to select the region of interest representing a portion of a subject to be measured.
The disadvantage of the known methods for detecting a region of interest for a camera-based remote vital sign measurement is that an occlusion of a predefined region of interest or a movement of the subject within the field of view may lead to the measurement of disturbance signals and to faulty measurements in general.